Field of the Disclosure
The present disclosure relates to methods of preventing wellbore formations from being subjected to heave-induced pressure fluctuations during tubular connections, well control procedures, and other times when the tubular is affixed to floating offshore drilling units.
Description of the Related Art
In wellbore construction and completion operations, a wellbore is formed to access hydrocarbon-bearing formations (e.g., crude oil and/or natural gas) by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, and/or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annulus is thus formed between the string of casing and the formation. The casing string is temporarily hung from the surface of the well. A cementing operation is then conducted in order to fill the annulus with cement. The casing string is cemented into the wellbore by circulating cement into the annulus defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
Deep water off-shore drilling operations are typically carried out by a mobile offshore drilling unit (MODU), such as a drill ship or a semi-submersible, having the drilling rig aboard and often make use of a marine riser extending between the wellhead of the well that is being drilled in a subsea formation and the MODU. The marine riser is a tubular string made up of a plurality of tubular sections that are connected in end-to-end relationship. The riser allows return of the drilling mud with drill cuttings from the hole that is being drilled. Also, the marine riser is adapted for being used as a guide for lowering equipment (such as a drill string carrying a drill bit) into the hole.
Once the wellbore has reached the formation, the formation is then usually drilled in an overbalanced condition meaning that the annulus pressure exerted by the returns (drilling fluid and cuttings) is greater than a pore pressure of the formation. Disadvantages of operating in the overbalanced condition include expense of the drilling mud and damage to formations by entry of the mud into the formation. Therefore, managed pressure drilling may be employed to avoid or at least mitigate problems of overbalanced drilling. In managed pressure drilling, a lighter drilling fluid is used to keep the exposed formation in a balanced or slightly overbalanced condition, thereby preventing or at least reducing the drilling fluid from entering and damaging the formation. Since managed pressure drilling is more susceptible to kicks (formation fluid entering the annulus), managed pressure wellbores are drilled using a rotating control device (RCD) (aka rotating diverter, rotating BOP, rotating drilling head, or PCWD). The RCD permits the drill string to be rotated and lowered therethrough while retaining a pressure seal around the drill string.
While making drill string connections on a floating rig, the drill string is set on slips with the drill bit lifted off the bottom. The mud pumps are turned off. During such operations, ocean wave heave of the rig may cause a bottom hole assembly of the drill string to act like a piston moving up and down within the exposed formation, resulting in fluctuations of wellbore pressure that are in harmony with the frequency and magnitude of the rig heave. This can cause surge and swab pressures that will affect the bottom hole pressures and may in turn lead to lost circulation or an influx of formation fluid. Annulus returns may also displaced by this piston effect, thereby obstructing attempts to monitor the exposed formation.